Alumina may be reduced to metallic aluminum by means of a carbothermic reduction process. This process is carried out at high temperature in an electric arc furnace.
The starting charge materials for this reduction process are solid particles of carbon and alumina. These materials are fed from the top of the furnace, collecting as a column and surrounding one or more carbon electrodes mounted within the furnace.
The actual reduction process takes place in the electric arc, at temperatures in excess of 2400.degree. C., thus, high temperatures prevail throughout the furnace.
There are several major problems caused by the nature of the charge materials to such a furnace. First, the bare carbon particles within the charge may complete an electrical circuit between the electrodes, whether there be multiple centrally located electrodes in the furnace or a single electrode, with the inner wall or hearth of the furnace acting as the second electrode. Completion of this circuit short circuits the furnace, reducing its efficiency.
Due to the high temperatures within the furnace, two additional problems result. The carbon particles may be air burned while in the charge column, reducing the available carbon and requiring an additional carbon input to overcome this burning process so that the required carbon level for reduction is met.
Also, temperatures approaching 2000.degree. C. at the bottom of the charge column may result in premature melting of the charge materials in the column, possibly causing the charge column to collapse.
It is desirable, therefore, to provide a charge for a carbothermic alumina reduction furnace which electrically insulates the carbon in the charge from the electrodes while in the charge column and which thermally insulates the carbon in the charge column, such that air burning of the carbon and premature melting of the charge are substantially reduced.